A typical radial tire includes a tread, a belt structure (“belts”) and a carcass. The carcass has an innerliner, a pair of inextensible beads, an apex (rubber filler) over each bead, two sidewalls, and one or more plies (“radial plies”). The plies have parallel reinforcing ply cords of typically nylon or polyester, which extend between, and wrap around, the beads.
Tire Making Process
In the tire making process, a green carcass (“green” meaning as yet uncured and still tacky) is built typically by wrapping a length of green innerliner and at least one radial ply over a “first stage building drum” and splicing the innerliner and ply ends together to form a cylindrical shape around the building drum. Two beads (each comprising a cable of steel filaments encased in green rubber) are then positioned over the carcass, one at each side. The portions of the ply that extend beyond the beads are then turned up (wrapped around) the beads, forming “ply turnups”. The resulting assembly, including the innerliner, ply, and beads, is called a green carcass. Then, green (uncured) sidewalls are applied around each side of the plies.
The green carcass is removed from the first stage building drum and mounted on a “second stage machine” where it is inflated to a toroidal shape, and its radially-outer surface is pressed against a green tread and belt package to form a “green tire”. In subsequent steps, the green tire is “stitched” (rolled with a roller) to remove air pockets and adhere internal surfaces together.
The green tire is then mounted in a curing mold, where a bladder is blown up within the tire cavity to press the tire's outer surface tightly against the mold's inner walls while the tire is vulcanized. In the mold, the tire's green rubber initially softens under heat but eventually cures (stiffens through polymerization) enough to be removed from the mold and allowed to cool outside the mold, where the curing reaction continues until the tire is cool. In some cases, the tire is inflated on a post-cure inflation stand (“PCI stand”) while cooling, to keep the tire shape uniform and the ply uniformly stretched, to prevent the ply from shrinking nonuniformly when the tire is still hot from the mold.
Uniformity Characteristics
After a tire is cured, it is typically tested for uniformity characteristics, such as radial runout, radial force variation, axial force variation, tangential force variation, and conicity, which are defined in the Definition Section hereinbelow.
Sources of Nonuniformity
Tire nonuniformity emanates from numerous factors in the tire making process, listed below in their order of occurrence in the tire building sequence:
Deformation Of Raw Components: The raw tire components (tread, sidewall innerliner, plies [ply cords], beads and belts) either are rubber or have a rubber matrix and are stored on long rolls in the deformable green state. So, the tire components may not remain uniformly thick during storage.
Nonuniform Placement On Building Drum: The ply cords may not be laid around the building drum with equal straightness and tension, and the two beads may not be positioned in a plane which is perfectly perpendicular to the drum (and tire) axis, or may otherwise not be parallel to each other over the ply on the building drum.
Nonuniform Placement On Second Stage Machine: On the second stage machine, if the belt and tread are not positioned symmetrically over the green carcass, the green tire, and hence the cured tire, will not be uniform. Also, later as the green rubber is blown up, the bead and ply positions can shift nonuniformly.
Components Shift In The Green Tire State: Before curing, the beads and plies are held in place only by their green rubber matrix and the surrounding green rubber. As the green tire is handled, the bead and ply positions can shift nonuniformly.
Nonuniform Mounting In The Mold: If the green tire is not positioned symmetrically within the mold, the finished tire will not be uniform.
Ply Splice: The ply is stiffer and heavier at its splice (where it is doubled due to the overlapping ply ends) compared to other locations.
Ply Stretching and Shrinkage: In the mold, the inflated bladder tensions (stretches) the ply outward, and heat shrinkage of the ply's nylon or polyester fibers tension the ply further. This tension (tensile stress) causes the ply to slip around the bead, but to a different extent at different locations around the bead, with a splice slipping around the bead least.
Nonuniform Curing: The rubber can “lock up” (stiffen under cure) around the ply at different times at different locations, thus locking in nonuniform ply stresses.
Tum Apparatus
After a tire is cured and cooled it is tested on a force variation machine (also called “tire uniformity machine”, abbreviated “TUM”, “tire uniformity inspecting machine”, and “tire uniformity apparatus”). Many patents describe TUM components and TUM designs, almost all of which share the same general principle of operation as follows:
The tire is mounted on a rotatable test rim. To ease mounting, the test rim is a “split rim” having two rim halves with flanges that come together to sealingly engage the tire's bead area. The tire is inflated and pressed against a rotatable load drum (also called “load-wheel”, “load roll” or “test drum”) whose axis is parallel with the tire axis. As the tire rotates against the load drum, force sensors (usually connected to the drum shaft) or displacement sensors measure changes in force (of tire against the drun) or displacement (of the tire surface from the nominal or at-rest tire surface location) in various directions (mainly radial and axial).
TUM designs vary as to whether the load drum rotates the tire or vice versa, tire rotational speed, which uniformity characteristics are tested, how to correct for deformities or nonuniformities in the test rim or load drum, how to correct for sensor errors due to TUM vibration, and how to correct for tire imbalance. Designs also vary on rim design and tire conveyance mechanism.
FIGS. 4A and 4B illustrate simplified results of a TUM test, using radial force variation (RFV) as an example. FIGS. 4A and 4B show radial force on the vertical axis 401 versus the tire's rotational angle from 0 to 360 degrees on the horizontal axis 402. FIG. 4A shows a force variation composite curve 405. The angular location 410 (corresponding to a circumferential location on the tread) of greatest force 411 represents a “hard spot”, where the tire presses hardest against the load drum. The angular location 420 of least force 421 represents a “soft spot”, where the tire presses least against the load drum. The force variation composite curve 405 can be “decomposed” as shown in FIG. 4B into a series (“Fourier series”) of constituent harmonic waveforms 431, 432, 433 for further mathematical analysis. The first harmonic 431 of radial force variation (abbreviated R1h) is also known as “radial runout.” A second harmonic waveform 432 and third harmonic waveform 433 are also illustrated.
With the data thus collected, there are a wide variety of methods for mathematically processing the force variation data to determine the need for uniformity correction and to determine the control parameters for correcting a tire on a uniformity correction machine (which may be the same as the TUM used to make the uniformity measurements).
Prior Art Correction Methods
Grinding
In the patent literature, the most commonly addressed method of correcting a uniformity characteristic is grinding off rubber from selected locations around the tread circumference (and/or possibly the tread shoulder or tire sidewalls). Numerous patents disclose a wide variety of grinding techniques, differing on how the grinder is interfaced with the TUM, when grinding occurs relative to testing, where laterally on the tread (shoulder, crown, etc.) to grind, and how to calculate from force variation data the grinding depth at each angular location (U.S. Pat. Nos. 5,022,186; 4,936,054; 4,736,546; 4,458,451; 4,173,850; 4,095,374; 3,948,004; 3,880,556; and 3,848,368). Disadvantages of grinding are that it contributes to environmental pollution and material waste, reduces tread life, and leaves an unattractive surface finish. Although grinding can eliminate dimensional nonuniformities, it is less able to alleviate internal stress nonuniformities.
Work Out the Nonuniformities; Hot from mold; Inflated
U.S. Pat. No. 3,529,048 discloses a method to improve stress uniformity of tire cords and to reduce circumferential variations in structural resistance to radial and lateral forces when rotating. A tire is mounted on a rim shortly after removal from the vulcanizing mold while being approximately the vulcanizing temperature, and inflated to typically 20-40 psig. While the tire cools, the tire is rotated against an applied load for one to two times the vulcanizing duration. In variations of the invention, the load can be axial against the tread, lateral against the sidewall, or oblique against the tread, shoulder or sidewall. In other variations, the load can be rolling contact (such as a rotating shaft) or sliding contact. The load surface can be curved (ex: shaft) or planar (ex: floor), smoothly cylindrical or contoured. There can be one or a plurality of shafts (loads).
Rotatingly Pressing Around Tire's Entire Circumference When Hot
U.S. Pat. Nos. 3,635,610; 3,529,048; 3,464,264 and 3,389,193 disclose various methods to improve uniformity characteristics, all based on rotating a cured or partially-cured tire while pressing it against a roller, to “run in”, “knead”, and/or “buckle” the tire's surface around its entire circumference, to alleviate nonuniform stresses. The patents differ as to whether this is done when the tire is still hot from the mold, reheated, or made hot by flexural heating. They also differ as to whether this is done while the tire is inflated or uninflated.
Post-Cure Inflation
Various patented methods (e.g., U.S. Pat. Nos. 4,420,453 and 2,963,737) of improving uniformity of a cured tire are based on “post-cure inflation” (“PCI” or “post-inflation”), defined as mounting a hot cured tire (soon after removal from the curing mold, before it has cooled down from the curing process in the mold) on a rim and keeping it inflated as it cools. The patented methods differ as to the inflation pressure, whether to spray-cool, and when to start and end the post-inflation. Although these processes are referred to as “post-cure” processing, in reality a tire generally continues to cure as it cools down after removal from the curing mold.
Heating Selected Tire Portions While Uninflated
U.S. Pat. Nos. 3,945,277; 3,880,556; 3,872,208; 3,865,527 and 3,632,701 disclose various methods of reducing nonuniformities and/or flat spots of a cured tire based on heating only selected portions of the tire while uninflated.
Rotating Uninflated After Molding
U.S. Pat. No. 5,853,648 discloses a device for cooling tires, which minimizes static stress. The tire is rotated in a vertical position, uninflated, while cooling after vulcanization.
Reduce Bead Spacing; PCI When Hot off the Mold
U.S. Pat. No. 3,039,839 discloses a method of solving tire shrinkage and distortion caused by shrinkage of nylon ply cords. The bead set (bead spacing when on a building drum) is narrowed (relative to the prior art) to increase the molding stretch during molding and impart a tire molding stretch to the cords. Upon removal from the mold, the tire is promptly mounted on an inflating rim (PCI stand) and kept inflated to typically 30 lbs until it cools to below the nylon cord's shrinkage temperature of about 200 F.
Ionizing Radiation
U.S. Pat. No. 3,838,142 discloses correcting radial force variation by irradiating soft-spot sections of the tread and/or sidewalls with ionizing radiation of high energy electrons.
Material Addition
U.S. Pat. No. 3,725,163 discloses reducing force variations by applying a small amount of adhering material to selected locations of the tread, which can be in the form of a spray or tape or applied by a marker.
Shims
U.S. Pat. No. 5,060,510 discloses correcting radial force variation by inserting circular ring wedges of circumferentially-variable thickness (serving as shims) between the rim and the tire's bead area.
Stretch Ply Cords
U.S. Pat. No. 5,365,781 (and its divisions U.S. Pat. Nos. 5,616,859 and 5,458,176) disclose a method and apparatus to correct uniformity characteristics in a cured radial tire by permanently deforming a portion of a carcass reinforcing member (i.e., selected ply cords) as a function of the magnitude of the uniformity characteristic. A significantly high inflation pressure permanently stretches the portion of the carcass reinforcing member beyond its elastic limit and permanently lengthens it to an extent that is inversely related to a restraint (applied by a pair of [sidewall] restraint rings [182 in FIG. 8]) at that location. Radial force variation and/or conicity may be corrected by varying the angles between the plane of each restraint ring and the plane of the tire sidewall and by applying differing amounts of restraint to the two sidewalls. A belt restraint ring (280 in FIG. 8) can be used to prevent the high inflation pressure from expanding the belt package. An alternate embodiment corrects force variation by mechanically (instead of by inflation pressure) stretching a portion of the carcass reinforcing member outwardly beyond its elastic limit.
Disadvantages of these methods are that the restraint rings are most capable of adjusting cord lengths for one hard spot and one soft spot which is approximately 180 degrees around the tire from the hard spot, however use of a cupped restraint ring (380) and multiple inflation pressure cycles is proposed for correction of composite radial force variation defects. Furthermore, the belt restraint ring can not force any kind of concentricity between the tread and the beads or the tire axis.
PCI With Variable Rim Width and Inflation Pressure
European Patent 888,872 discloses measuring a waveform of radial runout before vulcanization. In a first embodiment, immediately after vulcanization, the tire is post cure inflated (to elongate the ply cords) at a high temperature on a rim whose rim width is minimum at a location corresponding to the wave form peak. In a second embodiment, immediately after vulcanization, the tire is post cure inflated (to elongate the ply cords) at a high temperature, while restraining jigs are disposed against the tire shoulders (FIG. 10).
Pre-Cure Methods
Adjust Belt Position over Green Carcass: U.S. Pat. No. 3,926,704 discloses measuring conicity of a (green) unvulcanized tire and adjusting the location of the belts accordingly while on a tire building machine.
Adjust Tire Shaping when Green: U.S. Pat. No. 5,882,452 discloses measuring vertical deviation of a green tire from circularity while clamped on a green tire building drum, and then shaping the green tire into a complete circle according to the measured value.
U.S. Pat. Nos. 5,882,452 and 3,926,704 disclose methods of measuring and correcting nonuniformity before curing, and E.P. 888,872 discloses methods requiring measuring nonuniformity before curing. Such methods have the disadvantages of requiring uniformity measurement on each tire, requiring a corrective procedure that is specific for each tire, and the inability to detect/correct nonuniformities that arise during molding.